# Joints

One of the most appealing features of a physics engine is to simulate articulations. Articulations, aka. joints, allow the restriction of the motion of one body relative to another body. For example, one well-known joint is the ball-in-socket joint also known as the ball joint: it allows one object to rotate freely with regard to the other but not to translate. This is typically used to simulate shoulders of a ragdoll.

##### warning

The API for using Rapier joints with Bevy isn't very ergonomic right now because all joints must be stored inside of the JointSet resource (instead of being actual components). This will be improved in the future.

## Basic concepts#

Joints can be modeled in various ways but let's talk about the concept of Degrees Of Freedom (DOF) first. In 3D, a rigid-body is capable of translating along the 3 coordinates axes $\mathbf{x}$, $\mathbf{y}$ and $\mathbf{z}$, and to rotate along those three axes as well. Therefore, a rigid-body is said to have 3 translational DOF and 3 rotational DOF. We can also say a 3D rigid-body has a total of 6 DOF. The 2D case is similar but with less possibilities of movements: a 2D rigid-body has 2 translational DOF and only 1 rotational DOF (which forms a total of 3 DOF). The number of relative DOF of a body wrt. another body is the number of possible relative translations and rotations.

The goal of a joint is to reduce the number of DOF a body has. For example, the aforementioned ball joint removes all relative translations between two bodies. Therefore, it allows only the 3 rotational DOF in 3D simulations or the 1 rotational DOF in 2D simulations. Other joints exist allowing other combinations of relative DOF. Note that because there are less possible motions in 2D, some joints are only defined in 3D. This is illustrated by empty cells in the following table for joints that are not defined in 2D:

JointAllowed DOF in 2DAllowed DOF in 3D
Fixed jointNoneNone
Free jointAllAll
Prismatic joint1 Translation1 Translation
Revolute joint1 Rotation1 Rotation
Ball joint1 Rotation3 Rotations
Cartesian joint2 Translations3 Translations
Planar joint2 Translations + 1 Rotation
Cylindrical joint1 Translation + 1 Rotation (along the same axis)
Pin-slot joint1 Translation + 1 Rotation (along different axes)
Rectangular joint2 Translations
Universal joint2 Rotations

Currently, Rapier supports fixed, ball, prismatic, and revolute joints. Joints must be inserted into a JointSet. Each joint is attached to two distinct rigid-bodies identified by they rigid-body handles.

## Fixed joint#

A fixed joint ensures that two rigid-bodies don't move relative to each other. Fixed joints are characterized by one local frame (represented by an isometry) on each rigid-body. The fixed-joint makes these frames coincide in world-space.

##### info

Attaching multiple colliders to a single rigid-body will have the same effect as using one rigid-body per collider and attaching them with a fixed joint. However the multi-collider approach will be much more efficient and numerically efficient than the joint approach. So a fixed-joint should only be used when the multi-collider doesn't fit your use-case (for example if you want to read the force applied by the joint in order to break it dynamically).

let joint = FixedJoint::new(Isometry::identity(), Isometry::translation(0.0, -2.0));
commands.spawn().insert(JointBuilderComponent::new(joint, entity1, entity2));

## Ball joint#

The ball joint ensures that two points on the local-spaces of two rigid-bodies always coincide (it prevents any relative translational motion at this points). This is typically used to simulate ragdolls arms, pendulums, etc. They are characterized by one local anchor on each rigid-body. Each anchor represents the location of the points that need to coincide on the local-space of each rigid-body.

let joint = BallJoint::new(Vec2::ZERO.into(), Vec2::new(0.0, -3.0).into());
commands.spawn().insert(JointBuilderComponent::new(joint, entity1, entity2));

## Revolute joint#

The revolute joint prevents any relative movement between two rigid-bodies, except for relative rotations along one axis. This is typically used to simulate wheels, fans, etc. They are characterized by one local anchor as well as one local axis on each rigid-body.

let x = Vector::x_axis();
let joint = RevoluteJoint::new(Vec3::new(0.0, 0.0, 1.0).into(), x, Vec3::new(0.0, 0.0, -3.0).into(), x);
commands.spawn().insert(JointBuilderComponent::new(joint, entity1, entity2));
##### note

In 2D, revolute joints are ball joints are the same thing. Therefore, there is no RevoluteJoint type in 2D (use the BallJoint type instead).

## Prismatic joint#

The prismatic joint prevents any relative movement between two rigid-bodies, except for relative translations along one axis. It is characterized by one local anchor as well as one local axis on each rigid-body. In 3D, an optional local tangent axis can be specified for each rigid-body. If a local tangent axis equal to zero is specified, it will be computed automatically. Setting this tangent axis lets you control the fixed relative orientation of the rigid-bodies.

The prismatic joint supports the application of joint limits. This will restrict the relative distance between the rigid-body anchors (along the free joint axis) to remain in the specified range PrismaticJoint::limits. The signed distance is computed as (anchor2 - anchor1).dot(axis1).

let x = Vector::x_axis();
let mut joint = PrismaticJoint::new(Vec2::ZERO.into(), x, Vec2::new(0.0, -3.0).into(), x);
joint.limits = [-2.0, 5.0];
commands.spawn().insert(JointBuilderComponent::new(joint, entity1, entity2));

## Joint motors#

Ball, revolute, and prismatic joints support joint motors.

Motors allow you to make the linked rigid-bodies move relative to one another, along the free degrees of freedoms left by the joint, as if a motor was pushing them. The joint motor is simulated with a PD controller (Proportional Derivative controller) where you can set a target relative velocity along the free degrees of freedom as well as a target position. The stiffness of the PD controller controls the strength of the force that will be applied to make the bodies reach the target relative positions along the free DOFs. The damping of the PD controller controls the strength of the force that will be applied to make the bodies reach the target relative velocities along the free DOFs. All joints supporting motors have the following methods:

• configure_motor_position(target_pos, stiffness, damping) : this tells the joint motor that we want the relative position of the rigid-bodies along the free DOFs to be equal to target_pos , and that the relative velocity when it reaches that position should be zero.
• configure_motor_velocity(target_vel, damping): this tells the joint motor that we want the relative relative velocity of the rigid-bodies along the free DOFs to be equal to target_vel. There is no restriction on the relative position along the free DOF.
• configure_motor(target_pos, target_vel, stiffness, damping): this lets you control all the parameters of the motor's spring-like equation. This should be used only of the other configuration methods are not flexible enough.
• configure_motor_model(model): this selects the mathematical model for the motor's controller. All the available models use a spring-like equation. See the API documentation of SpringModel for details.

It is also possible to configure the maximum impulse applied by the motor: this limits the maximum force/torque the motor is able to deliver. The following examples show the configuration of the joint motor for a prismatic joint:

let x = Vector::x_axis();
let mut joint = PrismaticJoint::new(Vec2::ZERO.into(), x, Vec2::new(0.0, -3.0).into(), x);
joint.configure_motor_velocity(1.0, 0.5);
commands.spawn().insert(JointBuilderComponent::new(joint, entity1, entity2));